I remember we (porteus) were added to the menu in pendrivelinux. Perhaps this option works for you. It would be by far the simplest way to set up dual options from ISO files. I should add that i have not tested this.

In short, I extract the ISO's to subdirectories, e.g. /mnt/sdb2/32 and /mnt/sdb2/64, keep one copy of the /boot/ directory in the device root, and use the porteus.cfg there to point to the different subdirectories with the from_dev and from_dir cheatcodes.

It's difficult because you will eventually need to unpack the 32-bit Edition for boot files and after unpack the 64-bit Edition to make available the boot files for both arch. Maybe someone from the community can write a nice installer for Porteus